University of Helsinki, Faculty of Science, Department of Geosciences and Geography

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Taka, Maija

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2017-03-09T08:42:39Z

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2017-03-21

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2017-03-09T08:42:39Z

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2017-03-31

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URN:ISBN:978-951-51-2920-8

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http://hdl.handle.net/10138/176908

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Detecting trends in stream water quality is one of the key objectives of environmental monitoring. Identifying factors controlling stream water pollutants is challenging due to the diversity of potential sources, pathways, and processes. Natural processes regulating the water quality of a watershed are often affected by anthropogenic activities, resulting in the redistribution of runoff from base flow to storm flow and the introduction of new pollutant sources. Despite the observed consequences of urbanization, a lack of understanding of the factors simultaneously controlling water quality is among the biggest gaps in our current knowledge of hydrogeography. Moreover, prevailing discussions of land-cover effects often neglect the potential contribution of other factors, such as surficial deposits, in stream water concentrations.
This thesis aims to 1) examine the most influential watershed properties determining spatial variation in stream water quality; 2) identify key water quality and watershed variables controlling stream biotic responses (i.e. diatom community composition); 3) investigate the effects of multiscale temporal variation on urban runoff in cold climatic regions; and 4) evaluate whether advanced statistical methods are applicable in hydrogeographical modeling of small watersheds. To fulfill these objectives, spatial watershed-scale analyses were conducted using modern non-parametric approaches and theory-driven methods such as structural equation modeling. This thesis is based on unique data sets of both multibasin and multiyear sampling and spatial data from the Helsinki region, southern Finland.
A combination of GIS-based approaches and statistical analyses revealed significant links and novel insights into complex relationships between water quality and spatial biogeophysical properties of the surrounding landscape. The importance of land cover was emphasized throughout the thesis. Under base flow conditions the significance of soil type was mainly controlled by land cover. Further, this thesis demonstrates how land cover and stream water quality strongly determine the spatial assemblages of aquatic biota, as elevated pollutant levels were linked to decreased species richness and dominance of more tolerant species of diatom taxa. From a temporal perspective, the results suggest that urban runoff pollution is a chronic phenomenon, and is controlled by both runoff volume (summer) and pollutant sources (winter). Both the divergent temporal behavior and dominant role of diffuse pollution sources indicated challenges for stream water management practices. Based on the observed substance levels, year-round runoff treatment in urban areas is highly recommended. Finally, this thesis increases our knowledge of stream water quality variation in space and time. In this thesis, key local phenomena in contemporary hydrogeography were identified with a spatial modeling framework. The inclusion of indirect effects into the models improved our understanding of these systems, thus emphasizing the importance of simultaneously studying multiple concurrent processes.